108 THE SKULL [chap. 



3. In front of this is the maxilla (Mx), a still more con- 

 siderable bone. It not only forms the chief part of the outer 

 wall of the nasal cavity, but it also sends inwards a hori- 

 zontal plate, forming the middle part of its floor. At the 

 junction of its vertical and horizontal portions is the alveolar 

 border, in which the canine, premolar, and molar teeth are 

 lodged. 



4. The most anterior bone of this series is the premaxilla 

 (PMx), which also has an ascending or vertical plate, 

 forming the outer wall of the nasal cavity, and a horizontal 

 plate forming the anterior part of its floor ; at their junc- 

 tion its alveolar border lodges the incisor teeth. The pre- 

 maxilla forms the outer and lower boundary of the anterior 

 nares. \ 



Besides these four, there is a small bone which enters into 

 the outer wall of the upper part of the nasal cavity, between 

 the ascending process of the palatine, the maxilla and the 

 frontal. This is perforated by the duct, which conveys the 

 tears from the orbit into the nasal cavity, and is hence 

 called lachrymal. 



Above this a process from the frontal completes the 

 upper and posterior part of the outer wall of the olfactory 

 chamber. 



The floor of the nasal cavity is formed, as above said, by 

 the horizontal plates of the palatine, maxilla, and premaxilla 

 meeting the corresponding bones of the opposite side in the 

 middle line. The inferior surface of this same horizontal 

 layer of bone is the roof of the mouth, or bony palate. 



The roof of the nasal cavity is formed posteriorly by 

 the continuation of the frontal forwards beyond the cere 

 bral cavity, the " nasal process of the frontal" but mainl) 

 by a long narrow bone, the nasal bone (Na). The hinder 

 extremity of this lies upon the nasal process of the frontal 





